1. Field of the Invention
The present invention relates to processes for the desulfurization of petroleum products during the refining process or otherwise before combustion, and particularly to a laser-based method for the removal of sulfur, particularly in the form of dimethyldibenzothiophene, in hydrocarbon fuels.
2. Description of the Related Art
Clean desulfurization of hydrocarbon fuels is an important issue due to environmental concerns (green house effect, acid rain, ozone depletion) and compliance with the regulations set by international agencies, controlling the environment. Sulfur content in transportation fuel (diesel) is an environmental concern because upon combustion, sulfur is converted to SOx during combustion, which not only contributes to acid rain, but also poisons the catalytic converter installed in modem automobiles for exhaust emission treatment.
Due to these concerns, drastic changes and stringent regulations were implemented in many countries concerning diesel and gasoline. Currently the fuel specifications for all highway traffic in the U.S., Japan, and Western Europe limit the sulfur content of the diesel to be less than 500 ppm. The new regulations in many countries will further lower the contents of sulfur in diesel fuels. By the year 2006, the sulfur content in diesel has to be reduced to less than 15 ppm and to less than 30 ppm in gasoline.
For this purpose, various techniques, such as hydrogenation and caustic treatment, have been developed to reduce the sulfur contents in hydrocarbon fuels. These conventional hydro-desulfurization (HDS) methods can remove a major portion of the sulfur from diesel fuels, but they are unable to remove the so-called “hard sulfur”, i.e., the sulfur that is strongly bonded in a polycyclic aromatic sulfur compound. In order to meet the 15 ppm specifications for diesel in the future, hard sulfur contents, such as dimethyldibenzothiophene (DMDBT), must be removed from diesel and other feed stocks and products.
The conventional hydrodesulfurization (HDS) process for removing easy sulfurs and polycyclic aromatics has been adopted on a commercial scale. Easy sulfurs include non-thiophenic sulfur (elemental sulfur, disulfides, mercaptans, etc.), but not thiophenes, benzothiophenes, and dibenzothiophenes in which the substituents are away from the sulfur heteroatom. In the conventional HDS process, polycyclic aromatics with more than one aromatic ring are mostly reduced to polynuclear aromatics having a single aromatic ring (e.g., tetralins). Thus, there is a strong demand for removing hard sulfurs from a large number of polynuclear aromatics.
Conventional techniques have their own technical limitations and cost effectiveness (octane loss) to reduce certain sulfur compounds, such as 4,6 dimethyldibenzothiophene, which is a major obstacle to bringing down the sulfur level to <15 ppm limit.
Due to the above mentioned reasons, there is a continuing vital interest in the development of approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization. The procedures of sulfur removal are mostly related to degradation of the most stubborn sulfur-containing contaminants, which are benzothiophenes, particularly 4,6-dimethyldibenzothiophene, and they involve catalytic desulfurization and photolytic oxidation of this family of compounds.
Recent photochemical approaches for desulfurization of hydrocarbon fuels involve photochemical oxidation of sulfur-containing hydrocarbons by conventional UV or visible radiation sources (lamps). The sulfur-containing hydrocarbons are oxidized when suspended in aqueous-soluble solvent (e.g., acetonitrile), and the oxidation products are concentrated in this solvent due to their higher polarity. The exploration of these processes by Hirai et al., Ind. Eng. Chem. Res., Vol. 36, pp. 530-533 (1997) and Shiraishi et al., Ind. Eng. Chem. Res., Vol. 37, pp. 203-211 (1998); Vol. 38, pp. 3310-3318 (1999); Vol. 40, pp. 293-303; and J. Chem. Eng. of Japan, Vol. 32, No. 1, pp. 158-161 (1999) revealed the ensuing features of these processes:
(i) The photochemical excitation of benzothiophenes is diminished in the presence of naphthalene, which is due to triplet energy transfer from the photoexcited benzothiophenes to the ground state naphthalene;
(ii) Photo-oxidation is assisted by a triplet photosensitizer (9,10-dicyanoanthracene);
(iii) The desulfurization is improved by introducing hydrogen peroxide into the contact water phase, since H2O2 acts as a weak oxidizing reagent of the photoexcited benzothiophenes and also makes the triplet energy transfer between benzothiophenes and naphthalene less efficient.
Although these photo-oxidation approaches are efficient for removing sulfur from light oils, catalytically cracked gasoline, and vacuum gas oils, their application in industry is not obvious due to problems in the separation of the solvent, the oxidized products and the sensitizer. The products of photo-oxidation of benzothiophenes in water are benzothiophene carboxylic acids, and the major mechanism of photochemical degradation of dibenzothiophene in aqueous solution is the oxidation of a benzo ring to form benzothiophene dicarboxylic acid and opening the thiophene ring, leading to sulfobenzoic acids.
A somewhat promising process for application purposes appears to be a conventional photochemical desulfurization in a hydrogen peroxide aqueous solution extraction system that is suited for high sulfur-content-straight-run light gas oil and aromatic-rich light cycle. However, this procedure is performed through the use of a high-pressure mercury lamp for direct excitation of sulfur-containing compounds, and results in a decreased sulfur content only after very prolonged (36 hours) irradiation.
These photochemical desulfurization processes reported so far are unfortunately suppressed in the presence of aromatic compounds (2-ring aromatics) and are therefore too slow with substituted dibenzothiophenes. This finding is in line with the commonly considered relative feasibility of sulfur compounds to undergo desulfurization. The reactivities decrease in the order thiophenes>benzothiophenes>dibenzothiophenes.
None of the above publications, taken either singly or in combination, is seen to describe the instant invention as claimed. Thus, a laser-based method for removal of sulfur (DMDBT) in hydrocarbon fuels solving the aforementioned problems is desired.